Radio receivers are generally understood to comprise a front end and a back end. The front end of the receiver selects a desired signal from among the many frequencies collected by the radio's antenna and converts the selected signal to an intermediate frequency (IF) signal that can be easily processed by the back end. The back end extracts information from the IF signal provided by the front end.
The process performed by the radio receiver's front end is loosely referred to as tuning the receiver, and can be broken down into three sub-functions: 1) generating a mixing signal; 2) selecting a desired radio signal by filtering; and 3) mixing the desired radio signal with the mixing signal to generate an IF signal. While various methods of performing each of the functions can be used, most modern radio receivers use automatic tuning systems as described below.
The local oscillator (LO) signal is a signal that has a frequency selected so that when the LO signal is combined with a selected radio signal, the selected radio signal is converted to an IF signal. The LO signal is generated as follows. A frequency synthesizer provides a tuning voltage, commonly referred to as a VCO tuning voltage, to a voltage controlled oscillator (VCO). The VCO generates the LO signal having a frequency dependent on the value of the VCO tuning voltage. The LO signal is fed back into the frequency synthesizer, which checks to see if the frequency of the LO signal is correct by comparing it to a reference frequency derived from an accurate crystal oscillator. If not, the frequency synthesizer changes the VCO tuning voltage to adjust the frequency of the LO signal. The frequency of the LO signal is again checked, and the VCO tuning voltage is altered as necessary. This process continues until the output of the VCO, the mixing signal, is locked onto the appropriate frequency.
The desired radio signal is selected using tunable filters. The tunable filters use varactor tuning diodes similar to those used in the VCO, and have frequency responses that can be changed by application of a filter tuning voltage. When an appropriate filter tuning voltage is applied to the front end filters' tuning diodes, the desired radio signal is passed through the front end and delivered to a mixer, along with the signal from the VCO. The mixer then combines the LO signal and the selected radio signal to generate a constant IF signal suitable for processing by the back end of the radio receiver.
In most conventional automatic front end tuners, the necessary filter tuning voltage provided to each front-end filter is generated by using an 8 bit multiplying digital-to-analog converter (DAC) to alter the VCO tuning voltage for use by a particular front end filter. The VCO tuning voltage (the same tuning voltage supplied to the VCO as discussed above) is provided to a reference input of the DAC, and a digital value corresponding to the frequency of the tuned radio signal is retrieved from a non-volatile memory and applied to a digital input of the DAC. The DAC then generates the required filter tuning voltage.
One limitation of the radios described above, is that for the DAC to provide the tuning voltages required by radios used in different international markets, the DAC must be capable of applying a large range of different gains to the VCO tuning voltage. The necessary range of gains is achieved by using a large number of digital values, which are stored in the radio's non-volatile memory. To provide the number of different digital values needed to achieve the necessary range of gains, each of the stored digital values is represented as an 8-bit number. Relatively large amounts of non-volatile memory are needed to store all of the required 8-bit numbers, and large memory requirements can significantly increase the manufacturing cost of radio receivers.
In addition to the cost of the memory itself, large amounts of production time are often needed to determine what the 8-bit digital values should be, and to store those digital values in the non-volatile memory. A system that reduces the number of bits required for operation of the DAC could result in a significant savings in production costs by limiting hardware costs and setup time.